An analytical model is presented for predicting the effectiveness of rotating air-to-air energy wheels using only the characteristics measured on the same non-rotating wheels exposed to a step change in temperature and humidity. A relationship between the step change response and the periodic rotating response of an energy wheel is developed assuming that the energy wheel behaves as a first order linear system for water vapour and sensible energy exchange. This allows the prediction of the effectiveness of an energy wheel when only the characteristics of a stationary wheel exposed to humidity and temperature step responses are known. In Part I of this paper, effectiveness correlations and uncertainty bounds are developed for sensible and latent effectiveness of energy wheels using data from transient measurements. (c) 2005 Elsevier Ltd. All rights reserved.